1. Technical Field
The present invention relates to a structure and associated method of fabrication that reduces thermally induced strains in solder joints associated with solder balls that couple a ball grid array (BGA) module to a circuit card.
2. Related Art
A contemporary circuit board configuration may have a BGA module mounted on a circuit card. A BGA module comprises a substrate of dielectric material, such as ceramic or plastic, having both a top surface and a bottom surface. An array of solder balls are attached to a corresponding array of conductive pads on the bottom side of the BGA module, while one or more chips are attached to the top side of the BGA module. The pertinent circuit card is any pre-wired board comprising dielectric material and an array of conductive pads on the board. An example of a circuit card is a motherboard of a computer. The circuit card pads serve as attachment points for accommodating one or more BGA modules. Accordingly, a BGA module that is mounted on a circuit board has each solder ball attached to a conductive pad on the BGA module itself and also to a conductive pad on the circuit card. These conductive pads are respectively affixed to the dielectric substrate of the BGA module and the dielectric board of the circuit card. Thus, each solder ball is a structural element that is mechanically attached to dielectric sheets of material on each side of the solder ball.
When the circuit card is heated or cooled, the solder ball is subject to strains that arise from the differential rate of thermal expansion of the supporting dielectric structures. For example, a typical thermal expansion coefficient of the circuit card is 14 to 22 ppm/xc2x0 C. (ppm denotes parts per million), while a ceramic substrate of a BGA module may have a smaller thermal expansion coefficient of approximately 6 to 11 ppm/xc2x0 C. If the BGA module uses a plastic substrate material, the effective thermal expansion coefficient of the plastic substrate, at locations where a silicon chip constrains expansion of the substrate, is typically about 7 ppm/xc2x0 C. While the preceding materials, and corresponding thermal expansion coefficients, typify BGA substrates and circuit cards, materials could be characterized by a reversed relationship in which the BGA substrate has a higher thermal expansion coefficient than that of the circuit card to which the BGA module is attached.
Unfortunately, strains on the solder balls resulting from the aforementioned differential thermal expansion may cause fatigue failure in the BGA solder joints. U.S. Pat. No. 5,726,079 (Johnson, Mar. 10, 1998), which is hereby incorporated by reference, discloses an approach that could be used to reduce the effect of differential thermal expansion. With this alternative approach, a chip mounted on a substrate is encased peripherally with a dielectric material that is mechanically bonded to the substrate. Additionally, the chip is mechanically coupled to a conductive plate located above the top side of the chip, wherein the conductive plate comprises a material such as a stainless steel. Because of the mechanical bonding between the conductive plate and the substrate through the encased dielectric material, bending of the structure due to the differential thermal expansion is eliminated and the thermal expansion of the conductive plate mitigates the thermal expansion of the substrate of the BGA module. Thus the material of the conductive plate would be selected to have a thermal expansion coefficient that would eliminate the bending that arises from the mismatched thermal expansions of the substrate of the BGA module and the dielectric board of the circuit card. Although this method would greatly extend the BGA fatigue life, it would also increases the product""s cost because of the cost of the conductive plate and because of the process steps and associated equipment required to fabricate the encasing structure.
The present invention provides an inexpensive method, and an associated electrical structure, that reduces the thermally induced strain on solder balls of BGA modules, wherein the BGA module is mounted on a circuit card. The essence of the method is to increase the effective length of the connecting structure, between the BGA module and the circuit card, that deforms due to differential thermal expansion. This results in a reduction in strain in the connecting structure and, in particular, in the solder ball. Thus, increasing the length over which a given displacement acts effectively reduces the strain throughout the solder ball. In particular, the method of the present invention forms annular voids by removing material from the dielectric substrate of the BGA module so as to leave the solder balls affixed to regions of dielectric material, wherein the regions of dielectric material are each substantially surrounded by an annular void thus formed. Annular voids may be similarly formed around the regions of the dielectric board of the circuit card, wherein the regions are underneath the conductive pads of the circuit card to which the BGA module will be attached. Thus, the method forms peninsulas of dielectric regions if the formed annular voids substantially, but not completely, surround the dielectric regions. Alternatively, the method forms islands of dielectric regions if the formed annular voids totally surround the dielectric regions. A peninsula thus formed will be unseparated from the remaining dielectric substrate (or board), thereby leaving a thin connecting dielectric strip between the peninsula and the remaining dielectric substrate (or board).
Generally, the present invention provides a method for forming at least one electrical structure, comprising the steps of:
providing a substrate including a dielectric layer having a first surface, and at least one electrically conductive pad attached to the first surface; and
removing a first portion of the dielectric layer to form a void portion of the dielectric layer, wherein the void portion substantially surrounds a second portion of the dielectric layer, and wherein the pad is positioned on the second portion.
The present invention generally provides at least one electrical structure, comprising:
a substrate including a dielectric layer having a first surface;
at least one electrically conductive pad attached to the first surface; and
a void portion of the dielectric layer, wherein the void portion substantially surrounds a second portion of the dielectric layer, and wherein the pad is positioned on the second portion.
The present invention has several advantages. The present invention protects the integrity of solder joints to extend the fatigue life of BGA modules. The method of the present invention is inexpensive compared with other methods, such as that of U.S. Pat. No. 5,726,079 (discussed previously in Related Art section). Moreover, the present invention does not preclude using other methods and could be used in conjunction with other methods with little added cost.